I'm not exactly sure what you mean by "more busy." I'm going to assume you mean something more along the lines of 'more active.'
I'd assume they seem 'busier' because you have a myriad of information and capabilities at your fingertips --literally at your fingertips. Think about it... at any moment you can access anything and everything. Browse the web, check email, do something in an app, play a game, consume media on your iPod app, YouTube app, or 3rd party apps. I can't list all the possibilities, but I think you get the gist.
The iPhone is ultimate cure for boredom. If, at the moment, you're not engaged in some other everyday activity, you can just reach into your pocket and either communicate (via the phone, messages, email, social media, etc.), consume media (e.g. games, music, videos, podcasts, books, audiobooks, the radio, etc.), or even create content (e.g. snap photos/videos, edit photos/videos, do some writing, not to mention the hundreds of thousands of apps out there for other purposes).
So, yeah, people who have iPhones may seem a lot 'busier' due to the reasons I listed. The possibilities are virtually limitless. Therefore, there's always something you can do on your iPhone, so that may --and probably is-- the reason why they seem "more busy."